Thermal ink-jet printers which use heaters to dry ink on print media can also cause the print cartridge to be warmed significantly, since the heater is generally near the region of printing. This additional warming of the print cartridge causes unique problems in the operation of the printhead. Though the invention described herein was necessary only for one of the four inks used in a commercial color thermal ink-jet printer employing such a heater, it is generally applicable as a means of overcoming problems of high temperature operation on any ink-jet system suffering from the problems described.
There is a supply channel leading from the ink reservoir to each nozzle in an orifice plate. This supply channel, or ink feed channel, is carefully designed to provide a certain amount of resistance to flow. The optimal fluidic resistance balances the need for quick refill against the need for well-behaved (well-damped) refill dynamics. The fluidic resistance is necessary to provide sufficient damping of the ink in the nozzle during the refill portion of a drop ejection cycle. When a print cartridge is heated as described above, the ink in the printhead becomes less viscous. As a consequence, the fluidic damping is reduced, which decreases the stability of the ink refill process. In addition, the surface tension of the ink decreases as a function of temperature. These effects combine to cause the refilling ink meniscus to spill out onto the surface of the orifice plate, through which the ink is ejected from the printhead, and thereby form puddles. These puddles around the ink nozzles interfere with subsequent drop ejections.
Another consideration is that the warmer the printhead, the larger the drop that is ejected. When larger drops are ejected, the ink refill process starts with the ink meniscus in a more deeply retracted position. The combinations of unstable ink refill, low viscosity, and a deeply retracted meniscus makes the refill process susceptible to air ingestion. Ingested air bubbles interfere with subsequent drop ejection cycles, causing the next drop (or drops) to be either weak or missing.
Thus, what is required is a reconfigured printhead architecture that takes into account the foregoing considerations for thermal ink-jet printers employing a heating means to assist in drying ink printed onto a print medium.